The endocranial cast of Khirtharia (Artiodactyla, Raoellidae) provides new insights into the earliest evolution of the cetacean brain.

Introduction Raoellidae are small artiodactyls retrieved from the middle Eocene of Asia (ca - 47 Ma) and closely related to stem Cetacea. Morphological observations of their endocranial structures allow for outlining some of the early steps of the evolutionary history of the cetacean brain. The external features of the brain and associated sinuses of Raoellidae are so far only documented by the virtual reconstruction of the endocast based on specimens of the species Indohyus indirae. These specimens are however too deformed to fully access the external morphology, surface area, and volume measurements of the brain. Methods We bring here new elements to the picture of the raoellid brain by an investigation of the internal structures of an exceptionally well-preserved cranium collected from the Kalakot area (Jammu and Kashmir, India) referred to the species Khirtharia inflata. Micro-CT scan investigation and virtual reconstruction of the endocast and associated sinuses of this specimen provide crucial additional data about the morphological diversity within Raoellidae as well as reliable linear, surfaces, and volumes measurements, allowing for quantitative studies. Results We show that, like I. indirae, the brain of K. inflata exhibits a mosaic of features observed in earliest artiodactyls: a small neocortex with simple folding pattern, widely exposed midbrain, and relatively long cerebellum. But, like Indohyus, the brain of Khirtharia shows unique derived characters also observed in stem cetaceans: narrow elongated olfactory bulbs and peduncles, posterior location of the braincase in the cranium, and complex network of blood vessels around the cerebellum. The volume of the brain relative to body mass of Khirtharia inflata is markedly small when compared to other early artiodactyls. Conclusion We show here that, cetaceans that nowadays have the second biggest brain after humans, derive from a group of animals that had a lower-than-average expected brain size. This is probably a side effect of the adaptation to aquatic life. Conversely, this very small brain size relative to body mass might be another line of evidence supporting the aquatic habits in raoellids.